I was quite small and not very strong.

我很小而且不太强壮。

I just want to laugh and not think too much.

我只想笑，不想太多。

Something can still be beautiful and not perfect.

有些东西即使不完美也可以很美。

He knew summer was precious and not to be missed.

他知道夏天是宝贵的，不容错过。

Most students use it for fun and not for homework.

大多数学生用它来娱乐而不是做作业。

We must save the trees and not buy things made of ivory.

我们必须保护树木，不要购买象牙制品。

He drives averagely, not too fast and not too slow.

他开车平均地，不快也不慢。

Try to relax and not be so uptight.

尝试放松，不要那么紧张。

You need to speed up and not lag behind.

你需要加快速度，不要滞后。

Let's begin and not waste any more time.

让我们开始吧，不要再浪费时间了。